5 $ 
Dr. Brewster on new properties oj heat , 
Proposition VIII. 
The parts of the plate of glass which exhibit the two exterior sets 
of fringes, have the same structure as that class of doubly refract- 
ing crystals , including sulphate of lime, quartz, &c. in which 
the extraordinary ray is attracted to the axis , while the parts of 
the glass, which exhibit the two interior and the terminal sets, 
have the same structure as the other class of doubly refracting 
crystals, including calcareous spar, beryl, &c. in which the de- 
viation of the extraordinary ray from the axis, is produced by 
a : repulsive force.* The portions between these which produce 
the black spaces, have an intermediate structure, like those por- 
tions of muriate of soda, fluor spar, and the diamond, which are 
destitute of the property of double refraction. 
In order to establish this singular result, I combined a 
standard plate of sulphate of lime which polarised a bright 
blue of the second order, with the different parts of the glass 
which produced the six sets of fringes. When the axis of the 
plate of sulphate of lime was parallel to the fringes, or to CD, 
the blue of the second fringe in the first exterior set below 
MN, and the blue of the second exterior fringe above OP, were 
converted into black, but when the axis of the sulphate of lime 
was perpendicular to CD, the blue of the same fringes was 
converted into yellowish green. On the contrary, when the 
axis of the plate of sulphate of lime was perpendicular to CD, 
the blue of the second fringe of the first interior set above 
MN, and the blue of the second fringe of the second interior 
• See Laplace’s valuable Memoir, Sur la lot de la refraction extraordinaire dans 
les cristaux diapbanes, Mem. de L’lnstitut. 1809. 
